The present invention is related to the field of optical fiber communication; especially to wavelength division multiplexed optical communication systems.
In optical fiber communication, the wavelength division multiplexing is a method for increasing communication capacity with an optical fiber. In this method, a plurality of carriers with different wavelengths, each for a different WDM channel, is modulated with different binary modulation signals. These modulated carriers are multiplexed and transmitted with an optical fiber. At the receiver side, the transmitted carrier is demultiplexed to original wavelengths and the respective signals are detected.
In the 1550 nm wavelength band usually used for optical fiber communication, actually carrier frequencies with the spacing of 50 to 100 GHz are standardized. Because the frequency spacing of Δv=50 GHz corresponds to the wavelength spacing of about 0.4 nm, a wavelength demultiplexer with a resolving power of at least R=3871 (by Rayleigh criterion) is required. The multiplexer or the demultiplexer could be any one of the standard commercial devices like a grating, or a dichroic beam splitter, or an array waveguide grating, or a tandem of Fabry-Perot etalons, etc. The modulation rate, on the other hand, is getting faster for increasing the transmission capacity. The high bit-rate modulation of carrier in time regime causes spectral fringe broadening in the frequency regime because of shorter data pulses. Therefore cross talk is liable to occur between adjacent channels whose wavelengths are close to each other. Thus indefinite and simultaneous increase of WDM channel density (i.e. channel separation) and increase of data rate per channel (i.e. shorter data pulse) are not compatible.